1. Field of the Invention
The present invention., relates to a method for repairing a crack in an electromechanical rotor, a method for preventing crack growth in the electromechanical rotor, an electromechanical rotor and a rotary electrical machine where many slots, in which coils and wedges are inserted at the corresponding bottom portions and the corresponding top portions thereof, are provided at the periphery of the rotor core portion thereof along the axial direction so that in the slots, the coils can be fixed in the corresponding wedges.
2. Description of the Related Art
A rotor 300 of a conventional turbine generator will be described with reference to FIGS. 12-18.
FIG. 12 is a cross sectional view schematically showing a portion of the conventional rotor 300, and FIG. 13 is a partially cut away plan view schematically showing the rotor 300, perpendicular to the axial direction thereof. FIG. 14 is a perspective view schematically showing the fabrication state between a slot 303 and a wedge 305. FIG. 15 is a plan view showing a rotor shaft 301 under deformation. FIG. 16 is a perspective view showing the conventional rotor 300 of the turbine generator with cracks created at the rotor dove tail thereof. FIGS. 17 and 18 are plan views of the conventional rotor 300 for explaining a conventional repairing method for the cracks created at the rotor dove tail thereof.
As shown in FIGS. 12-14, the rotor 300 of the turbine generator includes the rotor shaft 301 and the core portion 302 formed integral with the rotor shaft 301. Then, many slots 303 are provided at the periphery of the core portion 302 thereof along the axial direction. Coils 304 are inserted into the bottom portions of the corresponding slots 303. Wedges 305 are provided on the corresponding coils 304 via corresponding insulating blocks 306 so as to be inserted into the ditches formed at the upper portions of the slots 303, respectively. In this case, the coils 304 can not be dropped off from in the corresponding slots 303 if the centrifugal force affects the slots 303 (coils 304) when the rotor shaft 301 is rotated.
The wedges 305 may be formed in any shape, but normally formed in dove tail as shown in FIG. 14. The wedges 305 may be formed in T-shape, Christmas tree-shape and the like. Since each slot 303 has some wedges 305, contacting edges 308 are formed between the adjacent wedges 305 at the corresponding contacting surfaces 307 between the wedges 305 and the slots 303. Surface pressures, originated from the centrifugal force, affect the contacting edges 308 and relative slips ±δ occur between the slots 303 (the core portion 302 of the rotor 300) and the wedges 305 when the core portion 302 of the rotor is rotated under the condition that the core portion 302 is curved due to the weight thereof or the bending vibration thereof by the curvature of “r”, as shown in FIG. 15. In this case, relatively large tensile stresses and compressive stresses may occur and concentrated at the sides of the core portion 302 of the contacting edges 308 so that some fretting damages occur at the stress concentrating areas of the contacting edges 308 and thus, some cracks occur at the same areas.
As shown in FIG. 15, supposed that the radius of the core portion 302 of the rotor is defined as “r0” and the length of the wedge 305 is defined as “L”, the core portion 302 is expanded and extracted at the wedge edge by the amount of δ which can be represented by the equation (1) when the core portion 302 is shifted to the upper point A or lower point B. In this case, since the wedge 305 is provided along the axial direction of the rotor shaft 301, the wedge 305 can not be expanded and extracted by itself. Therefore, the relative slip of 2δ occurs at least one of the contacting edges 308 between the corresponding wedge 305 and the core portion 302 every one rotation of a rotor shaft 301.δ=r0·L/2r  (1)
In this way, if the relative slip occurs at the contacting edge 308 under the condition that a relatively large surface pressure is applied to the contacting edge 308, the fretting damage occurs at the contacting edge 308 so that a crack 309 may occur at the contacting surface 307.
Moreover, the crack 309, which occurs at the contacting surface 307 of the core portion 302 of the rotor, may be grown by the bending stress generated when the core portion 302 is rotated under the condition that the core portion 302 is curved by the weight thereof or the bending vibration, the thermal stress due to the temperature difference between the outer side and the inner side of the core portion 302 at the operation of the turbine generator, or the residual stress in the core portion 302. Therefore, such a technique as removing the crack 309 created at the contacting surface 307 of the core portion 302 at the scheduled outage is disclosed (e.g., refer to U.S. Pat. No. 6,849,972).
With the conventional crack removing method as disclosed in Patent document No. 1, the surrounding area of the crack 309 is defined in dependent on the condition and the size of the crack 309, and removed, as shown in FIGS. 17 and 18. The surrounding area is turned into a crack removed area 310 after the removal.
Moreover, it is disclosed that some ditches for stress relaxation are formed at the corresponding contacting edge 308 in the side of the core portion 302 so that the tensile stress and the compressive stress due to the relative slip between the corresponding wedge 305 and the core portion 302 of the rotor 300 can not be concentrated onto the corresponding contacting edge 308 (e.g., refer to JP-B 4-29304(KOKOKU)). In addition, some techniques as mitigating or preventing the fretting fatigue at the contacting surface 307 in the side of the core portion 302 are disclosed (e.g., refer to JP-B 5-74304(KOKOKU), JP-B 7-40774(KOKOKU) and JP-B 7-44802(KOKOKU)).
With the conventional technique where the crack 309 created at the contacting surface 307 in the side of the core portion 302 of the rotor 300 is removed, the fine processing for the minute space inside the slot 303 is required so that the processing efficiency is deteriorated. With the conventional technique relaxing the concentration of the tensile stress and the compressive stress at the contacting edge 308 due to the relative slip between the wedge 305 and the core portion 302 or mitigating or preventing the fretting fatigue at the contacting surface 307 in the side of the core portion 302, some cracks due to the fretting fatigue may be mitigated or prevented, but no crack can be removed and the crack growth can not be prevented.